BEIRUT A leading international human rights group said Tuesday that evidence strongly suggests Syrian government forces fired rockets with warheads containing a nerve agent  most likely sarin  into a Damascus suburb in August, killing hundreds of people.

The report by Human Rights Watch was the latest voice to condemn Syrian President Bashar Assad's government for the alleged chemical attack on the sprawling, rebel-held Ghouta suburb on Aug. 21.

The attack brought the United States to the brink of a military intervention the Syrian civil war, now in its third year.

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The group said its activists were unable to go to Ghouta to collect remnants of weapons, environmental and bodily samples such as hair and blood to test for the chemical agent but that they sought technical advice from an expert on the detection and effects of chemical warfare agents.

To support its claims, the rights group said it analyzed witness accounts and "the type of rockets and launchers used" in the attack. HRW also said its experts studied documented medical symptoms of the victims and analyzed activist videos posted on the Internet after the attack.

Human Rights Watch released its conclusions in a 22-page report Tuesday.

Offering details which hadn't yet surfaced in evidence offered by the Obama administration or France -- both of which were quick to conclude that the Ghouta strike must have been carried out by Assad's government -- HRW said the rockets used in the Aug. 21 attack were "believed to be associated with the delivery of chemical agents":

Photos obtained by Human Rights Watch from Syrian activists show pieces of 330mm rockets which purportedly fell on the Ghouta suburbs of Damascus on Aug. 21, 2013, and a diagram of the type of rocket HRW says was used in the attack.
HRW

"Two separate surface-to-surface rocket systems believed to be associated with the delivery of chemical agents were identified. The first type of rocket, found at the site of the Eastern Ghouta attacks, is a 330mm rocket that appears to have a warhead designed to be loaded with and deliver a large payload of liquid chemical agent. The second type, found in the Western Ghouta attack, is a Soviet-produced 140mm rocket that, according to reference guides, has the ability to be armed with one of three possible warheads, including one specifically designed to carry and deliver 2.2 kilograms of Sarin." - Human Rights Watch

Symptoms of the victims from the attack "provide telltale evidence about the weapon systems used," Bouckaert added.

Assad: "Not (a) single shred of evidence" on chemical weapons use

Assad himself denied allegations that his forces used chemical agents in an exclusive interview Sunday with Charlie Rose. His regime has blamed the Syrian rebels, whom they call terrorists, for staging the attack to gain international sympathy.

Assad warned in the interview that there would be retaliation against the U.S. for any military strike against Syria.

"You should expect everything. Not necessarily from the government," he said when asked to elaborate, an apparent reference to the possibility the regime could unleash allied militant groups such as the Iranian-backed Lebanese militant group Hezbollah. He added, addressing the U.S., that it would "pay the price if you are not wise with dealing with terrorists."

Assad also denied that he was behind the attack, saying his soldiers were "in another area" at the time and insisting that no evidence has been presented.

Washington has cited intelligence reports as saying the attack on Ghouta killed at least 1,429 people, including more than 400 children, though other estimates are much lower.

The U.S. has been seeking international support for limited punitive strikes against Assad's regime, which it accuses of using chemical weapons in the assault. President Obama is seeking authorization from Congress for U.S. intervention in Syria.

Later Tuesday, Obama will also present his case to the American people in a nationally televised speech from the White House.

In a sudden shift, diplomatic efforts to end the bloodshed gained momentum Monday after Syrian Foreign Minister Walid al-Moallem welcomed a call from Russia, Damascus' close ally, to place Syria's chemical arsenals under international control.

Al-Moallem's remarks welcoming the Russian proposal, made during a visit to Moscow, were apparently the first official acknowledgment by Damascus that it possesses chemical weapons. However, it remains to be seen whether the statement represented a genuine goodwill gesture by Syria or simply an attempt to buy time.

U.S. Secretary of State John Kerry said earlier Monday that Assad could resolve the crisis by surrendering control of "every single bit" of his arsenal to the international community by the end of the week. Kerry reiterated the U.S. position that there is very compelling evidence that the Syrian regime used chemical weapons against his own people.